In the field of railway vehicles, regenerative brake control is widely used in which the main motor is made to operate as a generator during braking to produce a braking force and at the same time convert the kinetic energy of the vehicle to electrical energy and the electrical energy is fed back to the overhead wire. The electric power fed back to the overhead wire in the regenerative brake control can be used for power running of other vehicles, so that the power consumption can be reduced.
However, the regenerative brake control has two problems described below.
One problem is that, in the high speed range (equal to or higher than the constant torque terminal velocity), the regeneration capability is limited depending on the capabilities of the main motor and the inverter device, and a satisfactory braking force cannot be achieved.
The power of the main motor is determined by the voltage applied to the main motor and the current flowing to the main motor. In general, the voltage is determined by the power supply voltage supplied through the overhead wire, so that the current has to be increased in order to increase the power of the main motor. However, if the current increases, the heat generated by the motor and the inverter device also increases, so that the body of the main motor has to be enlarged to ensure sufficient cooling capacity, or the cooler of the inverter device has to be upsized. Furthermore, the number of semiconductor elements arranged in parallel in the inverter device may have to be increased. That is, the approach to increase the regenerative braking force in the high speed range by increasing the current flowing to the main motor involves increasing the size of the device and therefore the weight of the device, and thus is less effective to reduce the power consumption.
The other problem is that, in a situation where there are a small number of other power-running vehicles, the regenerative braking force has to be reduced in order to reduce the increase of the overhead wire voltage to protect the inverter device.
When there are a small number of other power-running vehicles, the power fed back to the overhead wire from the regenerative brake is not consumed, and therefore, the overhead wire voltage increases (this state will be referred to as a state of light load regeneration, hereinafter). As a result, the voltage applied to the inverter device can exceed the allowable value and damage the inverter device. To avoid this, the regenerative braking force has to be reduced to reduce the increase of the overhead wire voltage. As a result, the braking force becomes insufficient, and the shortage of the braking force has to be supplemented by the air brake, so that the power consumption cannot be sufficiently effectively reduced.
A technique for solving these problems is described in Patent Literature 1, for example. A drive device for a railway vehicle described in Patent Literature 1 comprises a motor, an inverter device that drives the motor, and a power storage device capable of charging and discharging and has a switch to selectively connect the power storage device in series with the inverter device (referred to as a serial type, hereinafter) or in parallel with the inverter device (referred to as a parallel type, hereinafter). The drive device further has a chopper circuit to cause charging and discharging of the power storage device. This configuration is intended to solve the problems 1 and 2 described above.
To address the problem 1 of the shortage of the regenerative braking force in the high speed range, the switch is operated to connect the inverter device and the power storage device in series with each other, thereby boosting the input voltage of the inverter device by the voltage of the power storage device. This can increase the voltage applied to the motor and thus the power of the motor, thereby increasing the regenerative braking force in the high speed range without increasing the current supplied to the motor (a high speed range electric brake function).
To address the problem 2 of the state of light load regeneration, the switch is operated to connect the inverter device and the power storage device in parallel with each other to activate the chopper circuit, thereby making the power storage device absorb part of the regenerated power (a regenerative energy absorption function).
In power running, the switch can be operated to connect the inverter device and the power storage device in parallel with each other to activate the chopper circuit, thereby causing the power storage device to discharge and supplying the electric power to the inverter device.